Dragons are mythical creatures. Most have wings, or can fly, which represents Zeus, the Sky God. Electric isn't very effective on Dragons, representing Zeus' thunder bolts.
I noticed all the simple types are not very effective on them as well (water, grass, fire). Is the only thing good enough to take on Dragons, other mythical creatures?
The Fairy type and other Dragons are super effective. Fairy's are myths. I have no idea how to explain Ice.
They can't have got the Dragon type from no where.